Diffractive optical elements (DOEs), lens arrays and refractive microoptical elements are becoming more and more important as elements of modern optics. Many electronic devices for consumer market applications nowadays often include also optical elements that are decisive for a key function of the electronic devices. As an example, mobile telephones with integrated digital cameras are becoming more and more popular nowadays. Also in optical recording apparatus optical elements for precise imaging and forming of light rays during reading and/or writing are necessary. In particular for applications for the consumer market the development of low cost production methods of optical elements having an adequate precision are very important.
In general, in hot-forming of optical surfaces one can distinguish between ‘pressing’ processes and ‘hot-embossing’ or ‘molding’ processes. When ‘pressed’, the shape of the surface of a substrate or of a semi-finished product for the optical element that is to be produced is changed in its entirety. As an example, a spherical or planar surface can be deformed into a particular aspherical surface. When ‘hot-embossed’ or ‘molded’, however, the profile of the surface of the substrate or semi-finished product is varied locally for the optical element. Thus, when producing diffractive optical elements or microoptic elements, i.e. structures of relatively small dimensions, the surface, which is in most cases planar or curved locally by a small amount only as compared to the microstructures to be produced, is preserved while local impressions or protrusions are hot-embossed or molded onto the surface. In comparison to pressing the deformation ratio for hot-embossing or molding is substantially lower.
Hot-forming of optical elements offers particular economic advantages, if finishing of the structures having an optical effect, i.e. of the diffractive or refractive structures, on the surface of the deformed optical element is not necessary any more. As is commonly known, this requires a high precision of the forming tools used for hot-forming. Diffractive optics, e.g. microlenses, require a surface quality of the order of a quarter of the wavelength of optical light. Diffractive microoptics should be produced even more precisely. One problem when producing optical elements by hot-forming is the inclusion of process gases in the surface of the optical element during the hot-forming process, as this results in disturbing crater-shaped impressions on the surface of the optical elements. From the prior art various measures are known for avoiding inclusion of process gases on the surface of optical elements during a hot-forming process.
According to a first approach hot-forming is performed in a vacuum chamber. As an example a high precision glass forming pressing apparatus is commercially available from Toshiba Machine Co., America, type GMP-211V, capable of exerting a maximum pressing force of 19.6 kN for a maximum outer diameter of the die of up to 110 mm under vacuum conditions of better than 6×10−1 Pa. This approach is, however, time-consuming and demanding for vacuum pumps.
According to another approach, which is disclosed in Japanese laid-open patent application no. 2002-293 553 A, a glass preform is produced by a pre-molding step with structures already formed on the surface of the glass preform that are deformed during a subsequent primary molding step into the desired structures. Due to the smaller deformation ratio the total volume of the process gas included between the die and the glass preform is smaller. However, the inclusion of process gas cannot be prevented reliably. Furthermore, an additional molding tool and an additional process step are required, which is less efficient.
EP 648 712 A2 discloses a process for press-molding of optical elements, wherein a blank is put into a press-molding form and a force utilized for pressing is increased and decreased periodically. While the pressure is increased that surface of the blank, which is in contact to the surface of the die, is deformed until a bubble of process gas is formed in a volume that is surrounded by the deformed surface area. When the pressure is reduced, the process gas, which is included in the volume and is under a certain overpressure, can escape laterally along the surface of the die. Due to the intermediate reduction of the pressure the total pressing time is longer. Furthermore, gas inclusions can hardly be predicted and modeled, so that it is very difficult to specify the precision of the optical elements to be produced in advance. Even if the pressure is increased and decreased various times periodically, residual inclusions of process gas persist, which is detrimental to the surface quality of the optical element.
U.S. Pat. No. 6,305,194 B1 discloses a process and apparatus for press-molding an array of optical elements. A relatively small ball of an optical material is put onto a shell-shaped central nest of a die. When the two dies are pressed against each other, the ball is more and more flattened. In this process the material flows radially outward and drives residual process gas out of the volume of the die. The material of the optical element is strongly deformed in this process, which results in relatively long processing times and high production costs. If relative tiny structures are formed on the surface of the die, e.g. for forming microlenses or diffractive structures, the flow of the material into the structures, e.g. into impressions, cannot be controlled during the process. Therefore one cannot prevent the inclusion of residual process gas into the surface of the optical element during the hot-forming process.
U.S. Pat. No. 6,305,194 B1 also discloses a method, wherein an upper half of a forming tool comprises a molding or hot-embossing portion, which is curved convexly, and wherein a lower half of forming tool comprises a molding or hot-embossing portion, which is curved concavely. A plano-convex preform is put in between the two halves of the forming tool, said preform being deformed to a concave-convex lens. The radius of curvature of the convexly curved molding or hot-embossing portion is smaller than the radius of curvature of the concavely curved molding or hot-embossing portion so that the respective molding or hot-embossing portion comes into contact with the preform near a central area, when the two halves of the forming tools are pressed against each other, which causes that no gas is trapped in the respective molding or hot-embossing portion, when the molten or softened material of the preform flows radially outward.
U.S. Pat. No. 6,305,194 B1 also discloses a process, in which an upper half of a forming tool comprises a convexly curved molding or hot-embossing portion and in which a lower half of a forming tool comprises a concavely curved molding or hot-embossing portion. A plano-convex preform is put in between the two halves of the forming tools, which preform is deformed to a concave-convex lens. The radius of curvature of the convexly curved molding or hot-embossing portion is smaller than the radius of curvature of the concavely curved molding or hot-embossing portion so that the respective molding or hot-embossing portion comes into contact with the preform at first near a central portion, when the two halves of the forming tools are pressed against each other, which causes that no gas remains trapped in the respective molding or hot-embossing portion, when the molten or softened material of the preform flows radially outward.
The deformation of the preform in this process is, however, relatively high. This results in a relatively long total processing time, which is not economical. Furthermore, the relatively high deformation ratio of the preform requires relatively high process temperatures, which causes stress within the optical element after deformation. Such stress or tension can be the reason for an undesired birefringence of the optical element. It can occur that the softened or molten material of the preform adheres to the respective molding or hot-embossing portion at the relatively high process temperatures required, which is detrimental to the optical quality of the optical element.